The present invention is directed to a polymer film dielectric capacitor configured for surface mounting to a substrate in an electronic circuit.
As the technology of surface mounting electronic components to substrates continues to gain acceptance in today's marketplace and continues to find preference over through-hole circuit board assembly, ceramic multi-layer capacitors are increasing in popularity at the expense of polymer dielectric capacitors.
Ceramic multi-layer capacitors, if not thermally shocked, will withstand high temperature solder assembly techniques to substrates. In such high temperature solder assembly situations, a ceramic multi-layer capacitor's value will shift upward with the temperature rise during soldering and then exponentially decay over a period of days with the rate of decay depending upon the specific ceramic body involved. Ceramic capacitors, since they are subjected to extremely high temperatures during their manufacture, do not require an insulated overcoat to protect them thermally so that they are small and their cost is kept low.
The present configuration of ceramic capacitors as they are adapted for surface mounting applications in the electronics industry has distinct shortcomings. One such shortcoming is the electrical terminal connection points presently used with ceramic multi-layer capacitors in surface mounted applications.
Presently available ceramic multi-layer capacitors configured for surface mounting applications have terminal arrangements which, when the capacitor is in assembled relation with a substrate upon which it is mounted, maintain the body of the capacitor only slightly above the level of the substrate itself.
This arrangement causes at least two major problems: First, the extremely narrow gap between the body of the capacitor and the substrate to which it is mounted makes it very difficult to flush out contaminants such as solder flux and the like which commonly accumulate during processing of electronic circuits in production volume operations. Second, and closely allied with the difficulty in flushing contaminants from between the capacitor body and substrate, is the problem that since the contaminants are generally inadequately flushed and therefore remain trapped between the capacitor and the substrate, there is thereby provided a leakage path across the capacitor terminals which can severely affect the performance of a capacitor within the circuit in which it is employed.
It is well known in the electronics industry that polymer dielectric capacitors, commonly known as plastic film capacitors, enjoy some important advantages over ceramic multi-layer capacitors. For example, insulation resistance, a measure of the capacitor's resistance to providing a leakage path, is generally higher with plastic film capacitors than with similarly valued and dimensioned ceramic multi-layer capacitors.
Further, the dissipation factor, a factor relating to the dissipation of energy by a capacitor, is generally lower with plastic film capacitors than with similarly valued and dimensioned ceramic multi-layer capacitors.
Of perhaps the greatest importance is the capability of plastic film capacitors to self-heal in the event of a short circuit. Plastic film capacitors are coated with thin metal electrodes (generally aluminum) which electrodes vaporize at a shorted area to instantly clear the short and self-heal. Ceramic multi-layer electrodes are thick film and will not vaporize and clear. As a result, the ceramic multi-layer parts, when shorted, can dissipate a large amount of energy, which energy is sometimes manifested as heat sufficient to start fires within equipment.
Thus, it would be of extreme value to the electronic industry if a plastic film capacitor, with all of its advantages over ceramic multi-layer capacitors, could be produced in a manner to render it compatible with today's surface mounting techniques.
The present invention is directed to an improved metallized film capacitor configured for surface mounting with electrical terminations which, when the capacitor is mounted to a substrate in an electronic circuit, maintain a substantial gap between the capacitor body and the substrate to which it is mounted, on the order of 0.005 inches, which greatly facilitates the flushing of contaminants from between the capacitor body and the substrate after processing, thereby greatly reducing the tendency of trapped contaminants to provide a leakage path across the terminals of the capacitor. The particular configuration disclosed herein has further advantages in that it enhances the physical integrity of the plastic film capacitor.
No presently extant application of plastic film dielectric capacitors to surface mounting techniques is known except for specially packaged, bulky devices incorporating thermally protective encasement techniques such as plastic boxes or bulky wrappings.
Yet a further advantage of the present invention is best understood when considering such prior art attempts to adapt plastic film capacitors to surface mounting applications. Prior art attempts to adapt plastic film capacitors to surface mounting techniques, as hereinabove discussed, involve provision of bulky thermal protection structures and a fairleading of electrical access terminals from the capacitor terminals within the thermally protected package to a point without the thermally protected package to which connection is made in employing the device in an electronic circuit. Typically, such thermally protectively packaged plastic film capacitors are uni-directional in their applicability. That is, such capacitors can only be mounted in a particular attitude and, if that attitude is not presented to the substrate, no effective electrical connection can be made. Such capacitors have a "top" and a "bottom" and if they are, for instance by some automated handling means, presented "upside down" for connection to a substrate in a production assembly operation, no electrical connection may be effected with the capacitor.
The capacitor of the preferred embodiment of the present invention, in contrast, has no "top" or "bottom"; it is multi-directional, and therefore it is more easily handled by automated assembly machinery, which results in a lower cost of manufacture of circuits involving capacitors of the type disclosed herein.